The present invention relates to a mechanical pencil, and more particularly to a mechanical pencil having an automatic lead extending mechanism in which the lead is extended, clamped and released merely by controlling (or increasing and decreasing) a pinching force applied to the pencil body.
According to the conventional mechanical pencil, when the lead is worn out during writing, it is extended by a turn system in which the mechanical pencil body is turned, a knock system in which the mechanical pencil body is knocked on the top, a side knock system in which the side button of the mechanical pencil body is pushed, a system of turning an end portion of the pencil body, or a system of pulling an end ring of the mechanical pencil body, or a system of pulling the end ring of the pencil body. However, these conventional systems are disadvantageous in that, before the lead is extended, the mechanical pencil body must be held again, or the fingers holding the body must be shifted; that is, the lead cannot be extended without such a troublesome operation.
On the other hand, the lead can be extended with the fingers holding the mechanical pencil body maintained unchanged in position according to the following systems: a tip knock system in which a tip end of the mechanical pencil is pushed against a writing sheet or the like, a shake system in which the mechanical pencil is strongly shaken up and down, and a bend system in which the mechanical pencil is bent in the form of the character "L". However, these systems in which it is unnecessary to shift the fingers holding the mechanical pencil to extend the lead are still disadvantageous in that, since it is necessary to shake the mechanical pencil with the wrist held above the writing sheet or to push the top of the mechanical pencil body against the writing sheet, the writing sheet may be become dirty or damaged, or abnormal force may be applied to the mechanical pencil to break or bend the latter.
In order to eliminate the above-described difficulties, a so-called "automatic lead extending type mechanical pencil" has been developed by the present inventors. In the mechanical pencil thus developed, a ball is interposed between the chuck and the tightening member; that is, the mechanical pencil of such a type is intricate in construction and must be manufactured with high accuracy. Therefore, it is disadvantageous in that it would require high manufacturing cost and is liable to malfunction. In addition, it suffers from difficulties that it is necessary to knock the mechanical pencil body to protrude the lead from the end thereof, or writing cannot be smoothly performed because its end pipe is kept pushed against the writing sheet.
In view of the foregoing difficulties, the present applicants have proposed a variety of pinch type mechanical pencils disclosed in, for example, U.S. Pat. Nos. 4,270,870 and 4,358,210. In each of the pinch type mechanical pencils thus proposed, substantially as in the case of an automatic lead extending type mechanical pencil, the lead can be extended merely by controlling (or increasing or decreasing) the pinching force applied to the pinching part of the mechanical pencil. However, they are still disadvantageous in that, since the pinching part does not cover all the circumference of the cylindrical mechanical pencil body, an operation of moving the fingers circumferentially of the mechanical pencil body while holding the latter to write cannot be carried out; that is, a so-called "turning and writing operation" cannot be performed, and also they are not so good in external appearance and design.
In order to overcome the above-described difficulties, a pinch type mechanical pencil has been developed in which a pinching part covers all the circumference of the cylindrical mechanical pencil body, and the external appearance is excellent. However, it is still disadvantageous in that, in order to depress the pinching part, it is necessary to apply a relatively great pinching force to the pinching part because of the resistance of the latter, and it is rather difficult to completely or positively restore the pinching part depressed to its original condition. A mechanical pencil of the type disclosed in U.S. Pat. No. 4,358,210 by the present applicants, suffers from the same disadvantages. Further, in such a mechanical pencil, the pinching operation cannot protrude the lead from the tip end pipe. Furthermore, that type of the mechanical pencil requires an intricate slider member.
In view of the foregoing, the present applicants have developed a pinch type mechanical pencil having a fluid pressure mechanism disclosed in U.S. patent application Ser. No. 008,243 filed on Jan. 29, 1987. It is excellent both in function and in design; however, it requires special techniques in manufacture due to a leakage of the fluid.